Posted Thu Oct 23, 2008, 1:21pm Subject: Stupid Question for the day. Why Pressurestats?
Continuing on my theme of beginning the day with at least one stupid question, figured today should be no different :)
As I understand it, pressurestats are used instead of thermostats in commercial/prosumer machines as they can effectively control the temperature of the machine more accurately than a thermostat since the former has a tighter deadband. Is this correct or have I, once again, lost the plot? What sort of temperature accuracy is generally expected from a Sirai pressurestat in good condition? While I've read about people using PIDs in HX machines, and can see the advantages of a completely electronic system v. electro-mechanical, as far as temperature accuracy is the PID a solution looking for a problem in this case?
Keepitsimple Senior Member Joined: 12 Sep 2006 Posts: 287 Location: UK Expertise: I like coffee
Espresso: Reneka Techno + moka pots Grinder: Mazzer Mini-e, Dualit 4 drip... Vac Pot: Cona "C" Drip: Melitta Thermo steel... Roaster: Given up with that !
Posted Thu Oct 23, 2008, 4:32pm Subject: Re: Stupid Question for the day. Why Pressurestats?
PetethePID Said:
Continuing on my theme of beginning the day with at least one stupid question, figured today should be no different :)
As I understand it, pressurestats are used instead of thermostats in commercial/prosumer machines as they can effectively control the temperature of the machine more accurately than a thermostat since the former has a tighter deadband. Is this correct or have I, once again, lost the plot? What sort of temperature accuracy is generally expected from a Sirai pressurestat in good condition? While I've read about people using PIDs in HX machines, and can see the advantages of a completely electronic system v. electro-mechanical, as far as temperature accuracy is the PID a solution looking for a problem in this case?
Pressurestats in a steam boiler respond very quickly when a steam tap is opened.
An electronic/PID temperature controller will undoubtedly be a more accurate and inherently more reliable engineering solution, but in the case of a machine with a small steam boiler you also need to have a rapid heater response to the call for steam engineered into the design.
Some machines with electronic temperature controls achieve this by switching on the heater immediately steam is demanded.
Posted Thu Oct 23, 2008, 8:57pm Subject: Re: Stupid Question for the day. Why Pressurestats?
PetethePID Said:
While I've read about people using PIDs in HX machines, and can see the advantages of a completely electronic system v. electro-mechanical, as far as temperature accuracy is the PID a solution looking for a problem in this case?
In a single (or double) boiler machine, a PID should yield much better control over the brew temperature, but this is not the case for an HX machine. Brew water is heated indirectly as it passes through the heat exchanger, and the brew temperature depends not only on the boiler temperature but also the idle time and flush duration. Obviously you don't need a PID to control steam temperature/pressure in the boiler; a pstat is fine. So I tend to agree: PIDing an HX machine is a bit of overkill.
PetethePID Said:
Continuing on my theme of beginning the day with at least one stupid question, figured today should be no different :)
JGG Senior Member Joined: 31 Mar 2006 Posts: 947 Location: Kentucky, US Expertise: I like coffee
Espresso: PID Silvia; PID Alexia Grinder: Mazzer Mini E (A), SJ; Rocky... Roaster: Hottop D w/PID; Thermometric...
Posted Thu Oct 23, 2008, 9:04pm Subject: Re: Stupid Question for the day. Why Pressurestats?
Hi, Pete -
There are two (almost) independent things at work here.
First, which property is best to measure? Your choices are pressure or temperature. For saturated steam, if you know one then you can compute the other. But they are measured differently.
Second, once you have measured something, should you use an all-electronic switching system for the heater circuit? Or an electro-mechanical system?
Pressure vs Temperature as Control Variable For a steam boiler with a lot of headspace full of saturated steam, there are advantages to measuring pressure as the control variable. This is at least partly because you will measure the exact same steam pressure irrespective of your probe placement. The pressure is the same everywhere within the gas phase. In addition, changes in pressure "lead" changes in temperature, so the controls can be more responsive.
Alternatively, you could measure the temperature in a steam boiler by inserting a sleeved thermocouple probe through the shell. The probe needs to have a certain minimum amount of "embedment" through the shell to avoid stem effects. But you probably don't want it in so far that it pokes into the liquid water. A probe in the steam is more responsive because the heat transfer is much more efficient. In addition, the liquid will be stratified and the temperature you measure would be different depending on depth. These are solvable problems, but don't exist if you are measuring pressure.
So I think that's why the steam boilers in HX machines usually use pressure as the control variable.
Now consider the "hot water boiler" on a single boiler machine. The contents of the boiler are exchanged constantly, and most of the water in the boiler is in the liquid phase. Every time you run the pump a pressure sensor would be fooled into thinking that the temperature had risen significantly. Also, the constantly changing conditions would probably mean that the relationship between pressure and temperature would be very unstable.
A major convenience of measuring temperature, though, is that you can usually get very good results with an external sensor. This means you do not have to go to the trouble of poking something through the shell of a hot water boiler.
So, not surprisingly, single boiler machines measure temperature as the control variable, and not pressure.
Electronic vs Electro-Mechanical In theory, once you've measured the control variable (either temperature or pressure), you could feed the information into an electronic controller, like a PID controller, and do all of the heater switching electronically. Almost any PID can just as easily accept analog signals from a pressure transducer as from a thermocouple or RTD.
As a practical matter, though, pressure transducers are expensive compared to thermocouples or RTD's. So most PID systems use temperature as the control variable simply because of the expense of turning pressure readings into voltage or current signals. In addition, thermocouples are among the most idiot-proof devices on earth.
Pressure, OTOH, can easily be sensed remotely by purely mechanical devices. You just "pipe" the pressure to the pressurestat. Since there is no fluid moving, there are no pressure drops along the way. And it is a very simple matter to use a diaphragm to sense pressure and flip a set of contacts back and forth. So it is very tempting to favor an electro-mechanical switching system when pressure is your control variable. The price(s) paid for all of this simplicity are noise, limited lifetime, difficulty in changing the setpoint, and the need for hysteresis (deadband) to reduce the number of contact cycles.
An ideal control system for a steam boiler, IMO, would be a PID controller and a pressure transducer. This gives all of the advantages of using pressure as the control variable, plus all of the advantages of electronic control.
For a hot water boiler (i.e. single boiler), I think a PID with a thermocouple sensor measuring temperature is hard to beat.
Posted Thu Oct 23, 2008, 9:47pm Subject: Re: Stupid Question for the day. Why Pressurestats?
JGG Said:
Now consider the "hot water boiler" on a single boiler machine. The contents of the boiler are exchanged constantly, and most of the water in the boiler is in the liquid phase. Every time you run the pump a pressure sensor would be fooled into thinking that the temperature had risen significantly. Also, the constantly changing conditions would probably mean that the relationship between pressure and temperature would be very unstable.
As a practical matter, though, pressure transducers are expensive compared to thermocouples or RTD's. So most PID systems use temperature as the control variable simply because of the expense of turning pressure readings into voltage or current signals. In addition, thermocouples are among the most idiot-proof devices on earth.
Pressure, OTOH, can easily be sensed remotely by purely mechanical devices. You just "pipe" the pressure to the pressurestat. Since there is no fluid moving, there are no pressure drops along the way. And it is a very simple matter to use a diaphragm to sense pressure and flip a set of contacts back and forth. So it is very tempting to favor an electro-mechanical switching system when pressure is your control variable. The price(s) paid for all of this simplicity are noise, limited lifetime, difficulty in changing the setpoint, and the need for hysteresis (deadband) to reduce the number of contact cycles.
For a hot water boiler (i.e. single boiler), I think a PID with a thermocouple sensor measuring temperature is hard to beat.
Hmmm, thanks guys, all interesting points. I think I can see how using a pressurestat would be a leading device in terms of managing temperature. Jim with regards the PID, the sensors themselves are cheap, but I believe until recently the cost of the PID itself was quite expensive. I agree though that even the little Silvia with a PID punches way above her weight in terms of espresso performance, certainly plenty of room for improvement but only at a very large cost increase.
Thanks for taking the time to answer my questions. Like my external pump one (also on this board), I Googled extensively before posting and was a little surprised the question doesn't come up more frequently.
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